The present invention pertains to a system for controlling an electric motor, and more particularly, to a system for using the E.M.F. across a motor armature to change the connection between a power source and the motor, when it is desired to change the motor between a motoring mode and a braking mode or between a braking mode and a motoring mode without bringing the motor to a stop.
In a variety of industrial applications it is desirable to be able to shift an electrical motor from a motoring (driving) mode to a braking mode and back to a motoring mode without bringing the motor to a complete stop. One such application is a mine portal car for use in underground mining operations, such as coal mining. Here, the car employs one or more electric motors to provide power for moving the car carrying mining personnel from the mine entrance into the mining area where the passengers disembark. Since the path between the loading area and the unloading area may not be level, the car may be accelerated for a time until it starts downhill where the car is then placed in a braking mode. It may then be desirable to accelerate the car again without coming to a stop. This process of accelerating and then braking may be repeated several times between one end of the run and the other end of that run. It is desirable to provide an electrical control circuit to alternately move the electric motor between the motoring mode and the braking mode without using mechanical contactors to carry the electrical current for the motor.